Electrographic imaging processes and techniques have been extensively described in patents and other literature. These processes may take the form of electrophotographic techniques whereby a photoconductive insulating material is first electrostatically charged and then imagewise exposed with light to form a latent image. Exemplary electrophotographic imaging processes are disclosed in U.S. Pat. Nos. 2,221,776; 2,277,013; 2,297,691; 2,357,809; 2,551,582; 2,825,814; 2,833,648; 3,220,324; 3,220,831; 3,220,833 and many others.
Electrographic imaging processes may also take the form of photoelectrographic techniques whereby a photoconductive insulating material is first imagewise exposed to form a persistent latent conductivity pattern from which multiple prints may be obtained by electrostatically charging and developing the persistent conductivity pattern using a variety of known electrostatic printing techniques. These processes have the advantage of requiring only a single light exposure that will produce many prints. This reduces both the time and cost of electrostatic reproduction. Exemplary photoelectrography processes such as that described above are disclosed in U.S. Pat. Nos. 3,879,197; 3,982,935; 3,512,966; 3,081,165; 4,033,769; 3,879,201; 3,859,089 and 4,661,429.
Acid photogenerators are known for use in photoelectrographic processes. For example, U.S. Pat. No. 3,879,197 to Bartlett, et al., describes an imaging process utilizing a material capable of forming a hydrohalide acid upon imagewise exposure to activating radiation. However, the hydrohalide acids formed in the Bartlett, et al., process are always used in conjunction with certain other organic addenda, tend to be somewhat volatile, and possess relatively low levels of conductivity.
In U.S. Pat. No. 4,661,429 to Molaire, et al., a photoelectrographic method is disclosed that utilizes a photoelectrographic element containing onium salt acid photogenerators. The method disclosed by Molaire takes advantage of the fact that exposure of the acid photogenerator significantly increases the charge decay in the exposed area of the layer. Imagewise irradiation of acid photogenerator layer will therefore create a differential charge decay between exposed and unexposed areas when coupled with electrostatic charging. The differential charge decay will create an electrostatic image that may be developed using known electrostatic printing processes.
Although the photoelectrographic elements of Molaire, et al., exhibit superior performance compared to many of the known photoelectrographic elements, they suffer from the disadvantage that they are sensitive to variation in the moisture content of the surrounding atmosphere. For example, as the relative humidity in the surrounding atmosphere increases, the photoelectrographic elements of Molaire, et al., become more conductive. Conversely, as the relative humidity in the surrounding atmosphere decreases, they become less conductive and more insulating. This change in conductivity is observed in exposed and unexposed areas of the photoelectrographic element to differing extents depending upon the specific formulation of the element.
For example, at high relative humidities, unexposed areas of a particular element cannot be charged adequately. As a result, the potential difference between exposed and unexposed areas will not yield a toned image of acceptable contrast. Conversely, at low relative humidity conditions, exposed areas of other photoelectrographic elements may not discharge to a level far enough below that retained on the unexposed areas of the element. Therefore, the difference in potential available for toning is again too small to yield images of acceptable contrast and quality. In sum, while a given formulation may perform adequately at some conditions, its electrical performance may change significantly in response to changes in relative humidity such that image quality becomes unacceptable.
Accordingly, it is an object of this invention to provide a photoelectrographic element that will not only provide persistent activity with good contrast and quality relative to each print but is also substantially insensitive to the widely varying changes in relative humidity which are encountered during normal photoelectrographic operating conditions.